The official CSCS Operatives (Green Card) test — practised here, free, unlimited retakes.
50 questions, 45 minutes, 47/50 to pass — same as the real sitting. Topics weighted to match the current CITB syllabus. Every answer comes with a plain-English explanation and a citation back to the HSE document or regulation it comes from.
Take it once cold for a baseline. Take it again on the topics you got wrong. When you can pass it twice in a row, you are ready to book the real one.